1. Technical Field
The present invention relates to a magnetic resonance imaging apparatus with an improved capability for use with a presaturation operation.
2. Background Art
In magnetic resonance phenomenon, the nucleus of an atom whose spin is not zero and which has a magnetic moment in a static magnetic field, absorbs and emits a specific electromagnetic wave resonantly. The nucleus resonates with an angular frequency .omega..sub.o shown in the following Larmor equation which is an equation defining the resonance condition including a relationship between the Larmor frequency .omega..sub.o, the gyromagnetic ratio .gamma. and the static magnetic field strength H.sub.o. EQU .omega..sub.o =.gamma..H.sub.o,
where .omega..sub.o =2.pi..nu..sub.o in which .nu..sub.o represents Larmor frequency. The Larmor frequency .nu..sub.o is a resonance frequency defined by the Larmor equation.
In accordance with an apparatus utilizing the above principle, the electromagnetic wave that is induced after the above resonance absorption and has a same frequency with the above, are received by a receiver coil and then signal-processed. Thereafter, diagnosis data which reflects a nuclear density, a longitudinal magnetic relaxation time T.sub.1, a transverse magnetic relaxation time T.sub.2, a flow, a chemical shift and so on are obtained without cutting an object to be examined, in particular, a slice image data of a biological body under medical examination is conveniently obtained.
In acquiring the diagnosis data by means of a nuclear magnetic resonance, a whole area of the biological body placed in the static magnetic field is excited and signal-acquired. However, on account of a limitation due to a structure of the apparatus and a clinical need for imaging data, a specific region is excited in real practice and is signal-acquired for the specific region.
In this case, the specific imaging region is, in general, a slice portion having a certain thickness. A nuclear magnetic resonance (NMR) signal such as an echo signal or FID signal from the slice portion is obtained by executing a data-encode process for plural times. The stands for Free Induction Decay which is an observed NMR signal following termination of the radiofrequency (RF) pulse, similar in shape to a damped sine wave. A group of such data is, for example, image-reconstructed by a second-dimensional Fourier transformation (2DFT) technique, so as to generate an image for the slice portion.
In the above-described MRI apparatus, a presaturation technique is performed in order to suppress a blood-flow artifact and motion artifact. As described in Japanese Patent Application Laid-Open 63-122439, the presaturation is performed prior to an imaging of the slice portion. An adjacent region that has a prescribed thickness and is positioned parallel to the slice portion, is selectively excited by a .pi.2 pulse so that a transverse magnetic component of that region is eliminated. As other techniques, there is available a method in which a surface orthogonal to the slice portion is presaturated. Conventionally, in the course of performing the presaturation, there are utilized a gradient magnetic field coil and a transmitter coil or transmit/receive (transceiver) coil. In recent years, the transmitter coil or the transmit/receive coil tends to be designed functionally and compact-sized in order to improve a S/N ratio therefor. Moreover, referring to FIG. 1, there are provided presaturation-use slices 4, 5 in upper end and lower end portions of the transmit/receive coil 1 such as a head coil and knee coil, respectively. There are also provided data-acquiring multi-slices 6 between the presaturation-use slices 4, 5.
However, in above type of transmitter coil or transmit/receive coil, particularly in the transmit/receive coil, the coil is so compact-sized that the region to be excited by the coil is very limited. In other words, since the transmit/receive coil 1 is so compact-sized that the excitation efficiency by the radiofrequency (RF) pulse is deteriorated in both the upper end portion 2 and the lower end portion 3 of the coil 1, thus the presaturation being not fully satisfactorily performed. Moreover, since the upper end portion and lower end portion are designated for the use with the presaturation, an image region is undesirably narrowed.